We propose to study the assembly of ribosomal proteins on nascent RNA precursors in Drosophila melanogaster during oogenesis and early development. It is anticipated that such studies will generate a better understanding of ribosome structure and synthesis. We intend to prepare sphere labeled antibodies to D. melanogaster ribosomes, ribosomal subunits, mixtures of ribosomal proteins and some single ribosomal proteins; and to use them to determine whether ribosomal proteins are present on nascent rRNA precursors; to determine the sequence of assembly of the large subunit proteins, the small subunit proteins, mixtures of proteins and some single proteins; and to determine the positions of single proteins on the surface of ribosomal subunits. The sequence of assembly will be determined by the localization of sphere labeled antibodies on RNP fibers of electron microscope spreads of active ribosomal RNA transcription units (Miller spreads). Antibodies will be localized on the surface of ribosomal subunits by the technique of ribosome - IgG - ribosome dimers (for other references, see Lake, 1976).